Electrical multiplying circuit



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June 4, 1946.- AIV. BEDFORDA V2,491,494

ELECTRICAL MULTIPLYING CIIAIRCUIT-v Filed Jan. 12, 1944 5 Sheets-Sheet 3 Patented .lune 4, 1946 UNiTE ELECTRICAL MULTELYNG CIRCUIT Alda Vernon Bedford, Princeton, N. J., assignor to Radio @orporation 'of America, a corporation oi' Delaware Application January 12, 1944, Serial No. 517,967

(Cl. lili- 1.5)

The present invention relates to secret communication systems and more particularly to an apparatus and method for the multiplication of voltages to obtain the product of two signals for use in such a secret system.

One form of such a communication system is shown in my copending application Serial No. 456,578, iiled August 29, 1942, while in the copending application of Frank P. Wipff, Serial No. 484,303, filed April 23, 1943, one form of multiplier circuit is shown for use in the foregoing system. In the system of my prior application, the speech or other communication signal S is multiplied by a coding signal K to obtain the product SK which is transmitted. At the receiver the incoming signal SK is multiplied by the reciprocal of the coding signal K to obtain the original signal S, that is, SK 1/K=S. In this system, the product SK is obtained by putting the positive cycles of either the signal S or the signal K on a carrier wave and then passing lt through a modulator tube which has its bias varied by the positive cycles of the other signal. The same thing is done with the negative cycles of th'e signals after they have been reversed in polarity. The several product components are then added together with the correct polarity to obtain the product SK.

An object of the present invention is to provide an improved and slmplied method of and means for multiplying two signal voltages by each other to obtain a product.

Another object oi the invention is to provide a multiplying circuit wherein devices have the property ofproviding an instantaneous voltage which is proportional to the square. of the instantaneous input| voltage for reasonable swing in a single polarity.

A further object is to provide a multiplying circuit wherein the spurious components of the output signal are reduced.

A further object is to provide an improved circuit for obtaining the product of two electrical signals, such as the mentioned signals S and K without having the signals S and K per 'se appear in the output. 4

A further objects of the invention is to provide an improved system for secret signalling. l

In the accompanying drawings,

Figure 1 is a block diagram of signalling apparatus embodying the invention.

Figure 2 is a circuit diagram of a reciprocal circuit that may be employed in the system of Figure 3 is a'circuit which is referred to in explaining the invention.

Figure 4 is a graph which is referred to in explaining the invention.

Figure 5 is a second graph which is referred to in explaining the invention.

Figure 6 is a circuit diagram 0f the preferred embodiment of the invention.

Figure 7 is a group of graphs which are referred to in explaining the invention.

In the several figures, similar parts are indicated by similzffreference characters.

Referring to Fig. 1, the invention is shown applied to radio apparatus that may be switched for operation either as a transmitter or as a receiver. The several switches are shown in the position for transmitter operation. This is the operation of the apparatus that will rst be described. A microphone and a speech ampliler are shown at 'l and 8, respectively. The signal S is applied'through a switch 9 to a multiplier unit Ii which preferably is designed in accordance with the present invention.` The code signal K may be produced byv means of a code disc I6, a mask il, a light source i3, a condensing lens I9, and a ph'otoelectric cell 2li. The signa1 K is supplied through amplifiers 2l and 22 and through a switch to the multiplier unit lil The resulting multiplier output signal SK, which is unintelligible, is supplied through an amplier 26 and through a switch 21 to a radio transmitter 28 or to a wire line, if preferred.

The shape of the waves S, K and SK are shown inFi`g.7.

The receiver for decoding the signal SKmay be the same apparatus as in Fig. 1 with switches 9, 23 and 2l thrown to their contact positions R. The multiplierl iii ls unchanged. Thesignal SK is supplied from a radio receiver 3| through the switch 9 to the multiplier I. The coding signai K produced by the disc i6 is supplied through the amplifier 2| to a reciprocal circuit 32 which may be designed in accordance with the invention described and claimed-in application Serial No. 484,304, filed April 23, 1,943, in the name of Carl A. Meneley, and entitled Reciprocal circuits.

vmultiplier lli. The resulting output cf multiplier I4 resembles the original communication signal S since SIc l/K=S, as shown by wave'S- in Fig. 7. The decoded speech signal S 1Samplied by the amplifier 26 and supplied through by suitable synchronizing means.

the switch 2i to headphones or to the loudspeaker 33.

The code disc l@ at the transmitter and the corresponding code disc at the receiver are held in synchronism and in the proper phase relation For example, at the transmitter a 60S-cycle per second current from a source 36 may be supplied through a switch 31 to a synchronous motor 33 which rotates the code disc i8. 'I'he 60C-cycle current also modulates a radio transmitter 39 for the trans'- mission of synchronizing signal to the receiver. At the receiver the switch 31 is in its contact position Rwhereby the received 60o-cycle current is supplied from a radio receiver 4i to the synchronous motor 38. Iny some cases it may be preferable to transmit the synchronizing signal over a wire line.

Fig. 2 shows, by way of example, one circuit that may be employed to obtain the reciprocal 4 flattened wave m may be obtained by selecting a non-linear resistor unit 43 having a suitable voltage-resiwance characteristic and, by adjusting the value of the resistor 42.

lSince the above-described reciprocal circuit is purely resistive whereby its operation is independent of frequency, if it is adjusted to produce wave l/K. This circuit is described and claimed in the above-mentioned Meneley application. The resistor 42 is of high enough resistance so that the driving source for the non-linear resistance unit Output of 01 there is only a slight variation in the current flow through unit 43. The unit 43 may consist of a .pair of copper oxide rectiiiers 44 and 45 connected to conduct current in opposite directions.

' ity across an anode resistor 41 and a portion of the resistor 448-49 of a second amplifier tube 50.

The rectangular wave n is producedby `applying the output of the tube 46 through a blocking capacitor 5I and a high impedance resistor 52 to a pair of diodes 53 and 54 which are connected to conduct in opposite directions. 'I'he resistors 55 and 53 of comparatively low resistance are connected in series with the diodes 5S and I4, respectively. A biasing voltage drop for opposing current flow through the diodes 53 and 54 is produced across the resistor 56 by connecting a source of voltage (not shown) thereacross, a resistor 51 being in series with the voltage source. Flow of current through the diodes causes this bias to become divided equallybetween diodes 53jand 54. Therefore the diodes 53' and 54 clip the applied wave m symmetrically about its A.-C. axis by becoming conducting on alternate cycles when the signal voltage exceeds the D.C. voltage drop across the' resistors 55 and 58, respectively. The resulting rectangular wave n is amplified and reversed in polarity by the tube B3. The rectangular wave n and the'fiattened wave 1n add in the portion of the anode resistors 43 and 49 that is common to the tubes 46 and lll t'o produce the desired reciprocal wave lx/K as shown in Fig. '1. m y

' If the wave m is flattened correctly and .ifthe waves m and .n are added with the correct relative amplitudes,4 the resulting signal will be substantially a true reciprocal of the wave K for a considerable amplitude range. The waves m the reciprocal of an applied wave having one wave form, it will then produce the reciprocal of an applied wave regardless of its wave form.

The multiplier of the present invention' makes use of a circuit having the property of providing an instantaneous voltage which is proportional to the square ofthe instantaneous input voltage for a reasonable voltage swing in a single polarity. Such circuits will be referred to as squaring circuits and will be designated Q where referred to in this specification. In the preferred form the waves S and K, to be multiplied( are added together with four different polarity combinations and squared in four different channels. Then the four squared signals are added with suitable polarities to obtainl the product SK in the output as illustrated in the following equa- 43 is of high impedance whereby 25 tions:

The term 'A15 the D.C. bias added to the .li-o. Waves to cause all the swing to have the same Il Il Il Il 5 `polarity with respect to the squaring device.

The squaring circuit here devised employs a plurality of small copper oxide rectifiers known A commercially as Varistors, the operation of which will be explained by reference to Fig. 3, wherein the varistor V5 is supplied with a voltage eo, which causes a current i to ow as indicated by the graph of Fig. 4. Because of the particular variable resistance of the varistor the current is substantially proportional to the square of the voltage for a reasonable swing in the positive.

direction. (This is not true for negative voltage.) A suitable series resistance R11 is used to give yan output voltage proportional to this current. It has been found that there is a value for the resistance of resistor R11 which provides an output voltage most nearly in 'accordance with a square law when a given varistor is used. Thus, by reference to Fig. 4. it will -be seen that the input voltage,e=ei{-A, which through the action of varistor V5 given an output voltage e: equal to (e1-FAW.

` Referring to Fig. 6 ofthe drawings, the preferred embodiment of the invention is shown wherein the audio-frequency signals to be multiplied are S and K. As shown, these signals are applied respectively to the grids of two ampliiler tubes 3| and 32, which have push-pull outputs S and S, and K and '-K.

In order that the desired sum voltages be obtained, the signals S and K are applied to a network of resistors in the following manner: Signals S'and K respectively traverse resistors Ra and R1 to provide a signal proportional to and n may be mixed with the correct relative I amplitudesby adjusting a variable tap E3 on the anode resistor 43. The correct shaping oi' the as designated in the circuit diagram. As shown, the network also includes resistors R1: and R15 leading respectively from points (S-K) and.

( S-i-K) to ground, and resistors R14 andlRis leading respectively from points (S4-K) and (-S-K) to the source of bias voltage which is applied at the two places +B1. An 8000 ohm resistance has been found satisfactory for the resistors R13, R14, Risand R15, while 100,000 ohm resistance has been taken as the value of resistors R5, Re, R7, Re, R9, R10, R11 and R12. i

The sum voltages at the four points of the network are applied with bias voltage'A and -A to four varistors V1, V2, Va, and V4 respectively, all of which control the current through the common resistor'il to give the product output SK. As

explained by reference to Figs. 3 and 4, the output across R11 is proportional to the sum of all the voltages which would have been generated if each varistor had supplied current to a separate resistor, as indicated by the foregoing squaring equations. It is to be noted that the varistors V2 and V4 are connected with opposite polarities from the varistors V1 and Va so that the D. C. bias voltages must be different. By reference respectively to the third and fourth equations it will be seen that the values (S+KA) and (S-K-A) are each preceded by another minus sign and included in brackets before squaring to properly ,indicate mathematically the effect of the reversed connection on these two varistors. These ve equations. show that, ideally, only the desired voltage SK is' produced in the output circuit of Fig. 6.

For compensating for small dissimilarities in the varistors and other circuit elements, it has been found desirable to provide variable resistors R1 and Ra for adjusting the relative amplitudes of -S and -K.

Where vacuum tubes have been utilized as squaring circuits in a secret communication system of the type here under consideration, it has been found that many spurious components are a part of the output signal and thereby rather seriously complicate the separation of the message signal from the impressed code signal. In the present invention, the spurious components of the output signal, caused by slight departure of the varistor characteristics from a true square law over an extended swing are so reduced as to be practically negligible. Thus, from Fig. 5, it can be seen that at any instant the voltages on the four varistors are distributed symmetrically with respect to the bias point A.v This can be shown to lend a property for suppressing the generation of spurious components. For example, if the curve of Fig. 5 has a square law at point A, but above A it exceeds a square law in steepness, and below A it rises at a rate slower than the square law, then the instantaneous current dut to component (S-i-K-l-A) will be too great, but that due to (-S-K+A) will be too small and thereby tend to cancel the error. Similarly, errors arising Vfrom inputs (S-K-l-A) and (-S+K+A) will tend to cancel.

, While in the foregoing the term "multiplying circuit has been used to define the novel circuit,

` it will be seen that the circuit actually is a sort o! modulator which is completely balanced in the sense that only the side band frequencies are prospectively responsive to two input waves, said devices functioning to add said waves in four `diierentpolarity combinations, means for applying voltages of predetermined polarity to the respective combinations, means for squaring said combinations respectively in four different channels, and means responsive to the voltage of said channels for producing an output voltage' proportional to the sum of said voltages, whereby the product of said incoming waves is obtained.

2. An apparatus for obtaining the product of a pluralityof voltages, comprising devices rel spectively responsive to two input waves, said devices functioning to add said waves in a plurality of different polarity combinations, means for applying' voltages of predetermined polarity to the respective combinations, means for squaring said combinations respectively, each 'in a different channel, and means responsive to the aggregate voltage of said channels for producing an output voltage proportional to the sum of said voltages,

whereby the product of said incoming waves isobtained.

3. An apparatus for obtaining the product of a plurality of voltages, comprising vacuum tubes in push-pull arrangement respectively responsive al to the sum of said voltages', whereby the product of said incoming waves is obtained said product being a multiplication of said wave magnitudes as measured from the alternating-current axes of said waves.

4. An apparatus for obtaining the product of a plurality of voltages, comprising devices respectively responsive to a message signal and a code signal. said devices functioning to add said signals in four diierent polarity combinations,

means for applying voltages of predetermined polarity to the respective combinations, means for squaling said combinations respectively in four different channels, and means responsive to the voltage of said channels for producing an output voltage proportional to the sum of said voltages whereby the product of said signals is obtained said product being a multiplication of duced, while the input frequencies and the'harmonies thereof are suppressed. v

Having thus described my invention, Irclaim: '1. An apparatus for obtaining the product of a plurality of voltages, comprising devices resaid signal magnitudes as measured from the alternating-current axes'of said signals.

5. An apparatus for obtaining the product of a plurality of voltages, comprising devices respectively responsive to two input waves, said devices functioning to add said waves in four different polarity combinations, means for applying voltages of Apredetermined polarity to the respective combinations, means including rectiers for squaring said combinations respectively in four different channels, and means responsive to the voltage of said channels for producing an output voltage proportional to the sum of said voltages,

whereby the product of said input waves is ob combinations, means including varistors for squaring said combinations respectively in four different channels, and means responsive to the voltageof said channels for producing an output voltageproportional to the sum of said voltages, whereby'the product of said input waves is obtained' said product being a multiplication of said wave magnitudes as measured from the alternating-current axes of said waves.

7. An apparatus for obtaining'th product of a plurality of voltages, comprising devices respec-` tively responsive to tv 1o input waves, said devices functioning to add said waves in four diierent polarity combinations, means for applying voltages of one polarity to certain Aof said combinations and voltages of opposite polarity to others of said combinations, means including varistors for squaring said combinations respectively in four different channels, and means responsive to the voltage of said channels for producing an 'output voltage proportional to the sum of said voltages, whereby the product of said input waves is obtained said product being -a multiplication ,of saidwave magnitudes as measured from the biasing voltage for producing an output A. C. wave having an instantaneous value proportional to the product of the corresponding values of said input waves said product being a multiplication of sa'id wave magnitudes as measured from the alternating-current axes of said waves.

9. The method of producing an output A. C. wave having an instantaneousvalue proportional to the product of the corresponding values of two input waves S and`K which comprises the step of separating S into +S and -S outputs, and separating K into -l-K and -K outputs, obtaining from said outputs the sum voltages (S+K),

.(-S-K), (-S-f-K), and `(S-K), applying bias voltages to the respective sum of voltages, squaring the respective resultant voltages, and controllingthe common output, whereby the A. C. Wave output is proportional to the sum of all the voltages.

10. The method of producing an output A. C. wave having instantaneous vvalues proportional to the corresponding values of two input waves S and K which comprises generating the waves -S and -K which are WavesfS and K, respectively, With reversed polarities; generating the sum voltage waves (S-l-K), (-S-K), (-S-l-K) and (SK); squaring each sum voltage wave; and combining the squared waves to form the product wave SK. ALDA VERNON BEDFORD. 

